Project 1: Consolidation of insulation materials

Cryogenic liquid storage tanks are constructed by a double-layer sphere. The interior storage contains the liquid. To insulate it from the outside temperature, the hollow space between the interior storage tank and the exterior container is filled with a powder material. Over the lifetime of the storage, periodic thermal expansion/contraction of the interior tank compacts the powder in the hollow space downwards, exposing the top and reducing the insulation effectiveness. We simulate this process by modeling the physical properties of the powder particles to evaluate the rate of compaction. (Click the picture to see an animation.)

Project 2: Modeling irregular shape particles

Design of construction equipment is challenging because the materials it has to handle is not as simple as a pure liquid. This is one of the major reasons for industries that deal with particulate materials to under-perform than their counterparts who deal with liquid materials. Lacking a reliable constitutive law to describe these granular materials, computer simulations with good descriptions of the particle’s physical properties can help enormously. We use overlapping spheres to study the effect particle shape on the bulk friction behavior by comparing with results from direct shear box physical tests. (Click the picture to see an animation.)

Project 3: Lunar soil processing

Construction on earth has a very long history prior to any scientific knowledge of the soil behavior. Its success has come from experience of many lessons learned from failures. To explore other celestial bodies and planets for resources, ingenious engineering based on scientific knowledge is needed. In this project we study the transport of granular materials such as a soil in hoppers and pipes. They represent the most basic industrial processing equipment for granular materials. Electrostatic forces between particles are considered under either earth or moon gravity. Can you guess which is which in the two figures on the right? (Click for the answer.)

Project 4: Rheological properties of granular materials

Everyone knows that the viscosity of a liquid is a well-defined property. It can be measured by several standard methods with off-the-shelve instruments. The most common one is a Couette-device. This device is a concentric cylinder with a narrow gap. The liquid to be measured fills this gap. The device has one fixed wall and one moving wall. A fixed speed is applied to move one wall and measure the required torque. The relation between the torque and the speed of the motion are then used to measure the viscosity. For granular materials, similar device has been constructed. It was found that the “viscosity” obtained was not only dependent on the materials, but also on the rate of shear and the solid fraction. In this study we investigate the effect of many micromechanical level parameters on the bulk behavior: particle size, shape, boundary condition, and the contact properties between particles. (Click the picture to see an animation.)

Project 1: Sea ice rheology

Sea ice is not a uniform homogeneous material. Its physical and mechanical properties depend on its salinity and temperature at the crystal scale and on its thickness and thickness distribution, floe size and size distribution, and concentration, at the geophysical scale. To be able to predict the motion and deformation of a sea ice cover, one must know how to relate the deformation to forcing. As shown in the pictures on the right, sea ice cover can range from a slush to an aggregate to various floes. These floe can be fairly large and concentrated until they simply pack together to form a consolidated sheet with extremely heterogeneous thickness distribution. Each of such sea ice cover has a different mechanical property. This study focused on the sea ice covers represented by a dispersed aggregate such as shown in pictures 2 and 3. Recognizing such composition of sea ice is simply a two dimensional granular material, one may using the granular materials approach to derive the stress and strain-rate relationship. The resulting rheology is more appropriate for the marginal ice zone with fragmented ice floes dispersed in open water. (http://onlinelibrary.wiley.com/doi/10.1029/JC092iC07p07085/abstract)

Project 2: Sea ice drift in a wave field

Motion of ice floes in the ocean is known to be driven by wind and current. The additional drift due to waves has been investigated in this project. The mechanism of how waves move a floating piece of ice is the same as the sliding of an object on a slope, provided that the ice floe is much smaller than the wavelength. For very large floes such as an ice berg, the mechanism is different. There the reflection of the wave momentum creates an equivalent force. The slope-sliding mechanism is compounded with drag and added inertia effects of the accelerating ice floe to determine it motion. While still oscillatory due to the wave motion, there is net forward drift of the ice floes. Being much larger than water molecules, this drift is different from the well-known Stokes drift, and is dependent on the floe size and its drag and inertia coefficients. (http://cedb.asce.org/cgi/WWWdisplay.cgi?129268).

Click here to see an animation

Project 3: Ice motion measured from AVHRR imagery

Remote sensing is a crucial part of sea ice study. It provides continuous observations over a large area of the earth surface. There are many different sensors of various capabilities onboard many satellites. In this study we use the Advanced Very High Resolution Radiometer (AVHRR) imageries to track the motion of ice features in the Sea of Okhotsk. AVHRR can provide images for cloud-free regions because it can sense from red to far-infrared bands. We use two consecutive images 12-hr apart of the same region. By an automated feature identification computer algorithm we can map out the displacement vectors over the entire ice field. This method was applied to the Sea of Okhotsk for several winters. A sample picture of the displacement field over the 12-hr span is shown on the right. Connecting these displacement fields over time, one can track the origin of ice at a location of interest. Statistical analysis of such information provides site evaluation of offshore structures and coastal impacts. (Click here to see a related publication.)

Project 4: Sea ice formation in a wave field

Cooling below the freezing temperature turns water to ice. In calm water such as over small lakes without the disturbance of wind, ice forms as a smooth sheet first, then crystal grows under the surface ice to form vertical structures called columnar ice. If wave is present, as in large lakes and the ocean, a different process takes over. Ice first forms into a slush with random crystal orientations. The slush congeals into pancake shape ice floes, which grow in time by incorporating the slush surrounding them and by freezing together with each other. The size of these pancake floes are strikingly uniform, as shown in the picture to the right (courtesy of Tony Worby, Australian Antarctic Division). In this project we investigate what controls this uniform size. Furthermore, under stormy conditions waves may raft the existing pancake ice floes together to mechanically thicken the ice cover. The thickening process is much faster than the thermal growth. However, thickening cannot proceed indefinitely because the resistance of the stacked up ice floes progressively makes the rafting more difficult. The equilibrium thickness of an ice cover under this rafting process is also investigated via laboratory and computer simulated experiments. (Click each picture to see an associate animation.)

Project 5: Sea ice and wave interaction

As wave propagates under an ice cover, it changes its velocity: both the speed and direction. This phenomenon is called refraction and is well-known in optics. In gravity waves, this phenomenon is related to the mechanical properties of the ice cover. On a geophysical scale, the mechanical properties of various ice covers are not characterized. Existing theories for gravity wave propagation under an ice cover either model ice cover as an elastic or a viscous material. In this study, a viscoelastic model is investigated. The purpose is to set up a simplest approach to unify the range of different ice types as shown in Project 1, where evidently picture 1 is like a viscous material and picture 4 is like a (imperfect) elastic material. The two in between very likely possess both material characteristics. In this project we determine not only the refraction property of gravity waves under different ice covers parameterized by a range of viscosity and elasticity, but also the attenuation of the wave energy, the reflection and transmission between ice covers of different viscoelastic properties. (Click here to see related publications.)

Project 6: Offshore structure under ice and wave actions

In this project we study the additional load, force and torque, due to impact of pancake ice on cylinders. Many offshore structures have successfully operated in open water conditions subject to wind, wave, and currents. In ice-infested regions, wave is reduced due to the attenuation effect from the ice cover, however, the motion of ice itself becomes a new concern. Since this wave induced ice actions are oscillatory, they may invoke resonance of the offshore structure. Even if the vibration does not pose threat to the structures, it is uncomfortable for workers. We apply a computer simulation as in Project 4 to study the impact frequency spectrum under different ice conditions. (Click here to see a related publication.) (Click the picture to see an animation.)

Project 7: Laboratory and field study of sea ice

Field study of sea ice is expensive because it requires large ships to travel to a remote location. Laboratory study is a valuable approach to obtain data under a well-controlled environment. We have used the Army Cold Regions Research and Engineering Laboratory (CRREL) and the Hamburg Ship Research Basin (HSVA) facilities from 1995 to 2008 to conduct a series of experiments. These experiments enabled us to identify the formation of pancake ice, the controlling factors of its growth dynamics, the rate of formation, and the wave propagation under pancake ice covers. On the right are two photos of the laboratory facilities at CRREL (the black and white one, click the photo to see a video) and at HSVA (the color one, click the photo to see a video). Photo of a graduate student (Maggie) in the sea next to an iceberg, and our research team (Dr. Toyota, two graduate students: Yong and Mingrui, and Hayley Shen) at HSVA.